FIELD OF THE INVENTION
This invention relates to internally manifolded and internally manifolded and internally reformed fuel cell stacks, and in particular, subassemblies of an anode/current collector/separator plate/current collector/cathode therefor which upon assembly with electrolyte provide wet seals between the electrolyte and the electrodes. The subassemblies provide ease of assembly resulting in reduced labor costs and long term stability and the separator plate design reduces the amount of material required for fabrication, in particular, of the main and feed rails comprising the wet seals, resulting in reduced material costs. In accordance with one embodiment, the separator plate design provides integration of the current collector and main rail and elimination of a separate feed rail.
This invention is particularly applicable to molten carbonates and solid conductor/solid oxide fuel cells.
Generally, fuel cell electrical output units are comprised of a stacked plurality of individual cells separated by inert or bi-polar electronically conductive ferrous metal separator plates. Individual cells are sandwiched together and secured into a single stacked unit to achieve desired fuel cell energy output. Each individual cell generally includes an anode and cathode electrode, a common electrolyte "tile" or "matrix", typically referred to as the active area components, and a fuel and oxidant gas source. Both fuel and oxidant gases are introduced through manifolds to their respective reactant chambers between the separator plate and the electrolyte tile. The area of contact between the electrolyte and other cell components to maintain separation of the fuel and oxidant gases and prevent and/or minimize gas leakage is known as the wet seal. A major factor contributing to premature fuel cell failure is corrosion and fatigue in the wet seal area. This failure is hastened by thin-film electrolyte corrosion of stainless steel surfaces of the separator plate at high temperatures and high thermal stresses resulting from differing thermal expansion characteristics between the separator plate and active area components during thermal cycling of the cell, causing weakening of the electrolyte tile structure through intracrystalline and transcrystalline cracking. Such failures permit undesired fuel and/or oxidant gas crossover and overboard gas leakage which interrupts the intended electrochemical oxidation and reduction reactions, thereby causing breakdown and eventual stoppage of cell current generation. Under fuel cell operating conditions, in the range of about 500.degree. C. to about 700.degree. C., molten carbonate electrolytes are very corrosive to ferrous metals which, due to their strength, are required for fuel cell housings and separator plates. The high temperature operation of stacks of molten carbonate fuel cells increases both the corrosion and thermal stress problems in the wet seal area, especially when the thermal coefficients of expansion of adjacent materials are different.
This invention provides fully internal manifolding of the fuel and oxidant gases to and from the individual cells of an assembled stack in a manner, due to the design of the cell components, which provides ease of assembly, long term endurance, stability of fuel cell operation, and a reduced number of individual cell components, thereby eliminating fit up problems between the various cell components.